What is the utility of nasal screen for methicillin-resistant Staphylococcus aureus (MRSA) in patients with skin and soft tissue infections?

In patients at high risk of MRSA infection (eg, prior history of MRSA colonization or infection, recent hospitalization/antibiotics, intravenous drug use, traumatic injury),1 particularly in the presence of an open wound or purulent drainage, a negative MRSA nasal screen does not rule out MRSA skin and soft tissue infection (SSTI), nor does a positive MRSA nasal screen reliably predict MRSA SSTI. In contrast, in low risk patients without severe disease, a negative MRSA nasal screen may be helpful in deescalating empiric anti-MRSA coverage.

The sensitivity of MRSA nasal screen by culture or PCR for SSTIs may be as low as 40%, higher among those with an ulcer (70%), with negative predictive values of 80% to 98% depending on the prevalence of MRSA in the population; its specificity is better (72% to 96%) with positive predictive values of 7% to 76%. 2

In a retrospective study involving 57 diabetic patients hospitalized with foot wound infection, the sensitivity of MRSA nasal screen was only ~40% with a negative predictive value of 80%. 3 Another study found a negative predictive value of ~90% for MRSA nasal screen among patients with a diabetic foot infection when MRSA isolation from wounds was uncommon (7.5%).4

Several reasons explain why patients with a negative MRSA nasal screen could still have MRSA SSTI, including colonization in other body sites known to harbor MRSA (eg, rectum, axilla, groin, oropharynx) 6-9 or direct wound contamination with MRSA in the absence of carriage, particularly in healthcare facilities.10

Bonus Pearl: Did you know that dogs, particularly those owned by healthcare workers, may also carry MRSA in their nostrils?.11,12

 

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References

  1. Stevens DL, Bisno AL, Chambers H, et al. Practice guidelines for the diagnosis and treatment of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America. Clin Infect Dis 2014; 59:e10-52. https://www.idsociety.org/practice-guideline/skin-and-soft-tissue-infections/
  2. Carr AL, Daley MJ, Merkel KG, et al. Clinical utility of methicillin-resistant Staphylococcus aureus nasal screening for antimicrobial stewardship: A review of current literature. Pharmacotherapy 2018;38:1216-1228. https://accpjournals.onlinelibrary.wiley.com/doi/abs/10.1002/phar.2188
  3. Lavery LA, La Fonatine J, Bhavan K, et al. Risk factors for methicillin-resistant Staphylococcus aureus in diabetic foot infections. Diabet Foot Ankle 2014;5:10.3402/dfa.v5.23575. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3984406/
  4. Mergenhagen KA, Croix M, Starr KE, et al. Utility of methicillin-resistant Staphylococcus aureus nares screening for patients with a diabetic foot infection. Antimicrob Agents Chemother 2020;64:e02213-19. https://pubmed.ncbi.nlm.nih.gov/31988097/  
  5. Currie A, Davis L, Odrobina E, et al. Sensitivities of nasal and rectal swabs for detection of methicillin-resistant Staphylococcus aureus colonization in an active surveillance program. J Clin Microbiol 2008;46:3101-3103. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2546770/
  6. Mermel LA, Cartony JM, Covington P, et al. Methicillin-resistant Staphylococcus aureus colonization at different body sites: a prospective, quantitative analysis. J Clin Microbiol 2011;49:1119-21. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3067701/#B4
  7. Baker SE, Brecher SM, Robillar E, et al. Extranasal methicillin-resistant Staphylococcus aureus colonization at admission to an acute care Veterans Affairs Hospital. Infect Control Hosp Epidemiol 2010;31:42-6. https://pubmed.ncbi.nlm.nih.gov/19954335/
  8. Manian FA, Senkel D, Zack J et al. Routine screening for methicillin-resistant Staphylococcus aureus among patients newly admitted to an acute rehabilitation unit. Infect Control Hosp Epidemiol 2002;23:516-9. https://pubmed.ncbi.nlm.nih.gov/12269449/
  9. Lautenbach E, Nachamkin I, Hu B, et al. Surveillance culture for detection of methicillin-resistant Staphylococcus aureus: diagnostic yield of anatomic sites and comparison of provider- and patient-collected samples. Infect Control Hosp Epidemiol 2009;30:380-82. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2665909/
  10. Boyce JM, Bynoe-Potter G, Chenevert C, et al. Environmental contamination due to methicillin-resistant Staphylococcus aureus: possible infection control implications 1997;18:622-7. https://pubmed.ncbi.nlm.nih.gov/9309433/  
  11. Boost MV, O’donaghue MM, James A. Prevalence of Staphylococcus aureus among dogs and their owners. Epidemiol Infect 2008;136:953-64. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2870875/#ref017
  12. Manian FA. Asymptomatic carriage of mupirocin-resistant methicillin-resistant Staphylococcus aureus (MRSA) in a pet dog associated with MRSA infection in household contacts. Clin Infect Dis 2003;36;e26-28. https://academic.oup.com/cid/article/36/2/e26/317343

 

 

Disclosures: The listed questions and answers are solely the responsibility of the author and do not necessarily represent the official views of Massachusetts General Hospital, Harvard Catalyst, Harvard University, its affiliate academic healthcare centers, or its contributors. Although every effort has been made to provide accurate information, the author is far from being perfect. The reader is urged to verify the content of the material with other sources as deemed appropriate and exercise clinical judgment in the interpretation and application of the information provided herein. No responsibility for an adverse outcome or guarantees for a favorable clinical result is assumed by the author. Thank you!

What is the utility of nasal screen for methicillin-resistant Staphylococcus aureus (MRSA) in patients with skin and soft tissue infections?

What is the sensitivity of nose swabs in detecting methicillin-resistant Staphylococcus aureus (MRSA) pneumonia?

In MRSA pneumonia, the sensitivity of nasal swab PCR may vary from as low as 24.2% to 88% (1-3). A 2018 meta-analysis found an overall sensitivity of 70.9% (community-acquired pneumonia/healthcare-associated pneumonia [HCAP] 85%, ventilator-associated pneumonia 40%) with overall negative predictive value of 96.5% (based on an overall MRSA pneumonia prevalence of 10%) (4). 

A single center  study involving  patients with possible HCAP and a low clinical pulmonary infection score (CPIS) — for whom antibiotics may not be necessary anyway (5)—suggested that discontinuation of empiric vancomycin in patients without an adequate respiratory culture and a negative nose and throat culture may be reasonable (6).

However, a prospective study of ICU patients concluded that “clinicians cannot reliably use the results of initial negative MRSA nasal swab results to withhold empirical MRSA coverage from patients who otherwise are at risk for MRSA infection” (3).

The previously cited 2018 meta-analysis study (4) cautions against use of MRSA screening in patients with structural lung disease (eg, cystic fibrosis or bronchiectasis) because colonization may be more frequent in the lower respiratory tract in these patients and screening tests may therefore be discordant (4).

Collectively, the available data suggest that it is reasonable to use a negative MRSA screen to help exclude pneumonia due to this pathogen in patients in whom MRSA infection is not highly suspected or those who are not severely ill.

 

References

  1. Rimawi RH, Ramsey KM, Shah KB, et al. Correlation between methicillin-resistant Staphylococcus aureus nasal sampling, and S. aureus pneumonia in the medical intensive care unit. Infect Control Hosp Epidemiol 2014;35:590-92. https://www.ncbi.nlm.nih.gov/pubmed/24709733
  2. Dangerfield B, Chung A, Webb B, et al. Predictive value of methicillin-resistant Staphylococcus aureus (MRSA) nasal swab PCR assay for MRSA pneumonia. Antimicrob Agents Chemother 2014;58:859-64. https://www.ncbi.nlm.nih.gov/pubmed/24277023
  3. Sarikonda KV, Micek ST, Doherty JA, et al. Methicillin-resistant Staphylococcus aureus nasal colonization is a poor predictor of intensive care unit-acquired methicillin-resistant Staphylococcus aureus infections requiring antibiotic treatment. Crit Care Med 2010;38:1991-1995. https://www.ncbi.nlm.nih.gov/pubmed/20683260
  4. Parente DM Cunha CB Mylonakis E et al. The clinical utility of methicillin-resistant Staphylococcus aureus (MRSA) nasal screening to rule out MRSA pneumonia: A diagnostic meta-analysis with antimicrobial stewardship implications. Clin Infect Dis 208;67:1-7.
  5. Napolitano LM. Use of severity scoring and stratification factors in clinical trials of hospital-acquired and ventilator-associated pneumonia. Clin Infect Dis 2010;51:S67-S80. https://www.ncbi.nlm.nih.gov/pubmed/20597675
  6. Boyce JM, Pop O-F, Abreu-Lanfranco O, et al. A trial of discontinuation of empiric vancomycin therapy in patients with suspected methicillin-resistant Staphylococcus aureus health care-associated pneumonia. Antimicrob Agents Chemother 2013;57:1163-1168. http://aac.asm.org/content/57/3/1163.full.pdf
What is the sensitivity of nose swabs in detecting methicillin-resistant Staphylococcus aureus (MRSA) pneumonia?